Interpretive Summary: A simple and rapid technique termed refractometry was evaluated to measure the total amount of blood proteins in hybrid striped bass over a wide range of salinities. The total amount of blood proteins can be used as a broad clinical indicator of health, stress, and well being in fish. Routine measurement of the total blood protein can be used as a tool for making management decisions and monitoring the status of both wild and cultured populations. Many methods exist for determining these proteins, but require lengthy analysis times, require expensive equipment and expertise, and create chemical disposal problems. Refractometry which avoids these problems is used in land based animals, but may not be suitable for fish because of changes observed in other blood components in fish held at different salinities. Blood components, including total protein were evaluated in hybrid striped bass adapted to a wide range of salinities and the refractometry method was tested against another standard method for measuring total blood proteins in fish. Salinity affected many blood components, and although the changes were small the values were always higher using the refractometry method. Additionally, the difference in values between the two methods became greater as the salinity increased. It was concluded the refractometry method is not suitable for measuring accurate total blood protein levels in fish over a wide range of salinities.

Technical Abstract:
Total plasma protein (TPP) is a broad clinical indicator of health, stress, and well being. A simple and rapid technique for determining TPP is refractometry, which measures the refractive index of all dissolved materials in solution. It was hypothesized plasma dissolved solids in fish held at increasing salinity levels would result in plasma samples with a higher refractive index and affect true TPP readings. Efficacy of refractometry over a range of salinities was evaluated by comparing TPP values obtained by refractometry with those obtained by a dye-binding assay (Biuret method) specific to proteins. Blood analytes were evaluated in hybrid striped bass Morone chrysops x M. saxatilis (HSB) adapted to nominal salinities of 0, 10, 20, and 30 g L-1. Hybrid striped bass adapted to hypertonic environments exhibited higher (P<0.05) electrolytes, TPP, and osmolality than those adapted to low salinity, but were still within the normal reference interval for HSB. Although salinity affected TPP determined by both methods, the observed increase in TPP values over the range of salinities was significantly greater in samples analyzed by refractometry. Hemoconcentration may partially explain increases in TPP with salinity, but salinity also influenced the refractive index in samples collected from saltwater relative to freshwater adapted HSB. Total plasma protein concentrations measured by refractometry were always higher and the difference in absolute values determined by the two methods appears to be caused by a factor not measured in this investigation. Moreover, while there is good agreement in TPP values measured using either serum or plasma samples, plasma may give a more accurate determination of TPP.